Light emitted from a point of an object and passing through an optical system cannot be converged to one point due to diffraction or aberration of the optical system and therefore has a slight spread. A light distribution with such a slight spread is called a point spread function (PSF). An image produced by image capturing using an image capturing optical system corresponds to one formed by convolution of an ideal object image with the PSF and thus has a degraded resolution (blur).
For this reason, a resolution enhancement process to correct such a degraded image is often performed. One example of the resolution enhancement process is an image restoration process. The image restoration process is performed by, for example, convoluting an image to be corrected (input image) with an image restoration filter having an inverse characteristic to that of an optical transfer function (OTF) of the image capturing optical system. The OTF can be acquired by performing a Fourier transform on the PSF.
However, the image restoration process may amplify a noise generated in an image capturing apparatus (image sensor) used for the image capturing and therefore contained in the degraded image or generate ringing. For this reason, it is difficult to produce a good high-resolution image with the resolution enhancement process by the image restoration process simply using an image restoration filter having a characteristic inverse to that of the OTF. An image restoration method that can solve such a problem is known which uses a Wiener filter capable of controlling a degree of restoration depending on an intensity ratio (SNR) of an image signal to a noise signal. The Wiener filter is provided with an adjustment term (amplification factor adjustment term) to control an amplification factor of the noise.
However, setting the amplification factor adjustment term of the Wiener filter such that the amplification factor increases can improve a resolution enhancement effect of the image restoration process, but significantly amplifies the noise. On the other hand, setting the amplification factor adjustment term such that the amplification factor decreases can suppress the noise amplification, but decreases the resolution enhancement effect.
As methods of performing a noise reduction process on a high-resolution image produced by the resolution enhancement process to produce an image having a high resolution and less noise, Patent Literature 1 and Non-Patent Literature 1 disclose methods using a wavelet transform. The method disclosed in Patent Literature 1 reduces, in a wavelet space, a noise generated when a blur correction is performed as the resolution enhancement process. The method disclosed in Non-Patent Literature 1 performs, when an image restoration process as the resolution enhancement process is performed on an entire image, a noise reduction process uniformly on the entire image in the wavelet space on a basis of a frequency characteristic of the resolution enhancement process to reduce a noise amplified by the resolution enhancement process